Field 'executionTimeMillis' missing from explain execution stats output

assert: undefined is not greater than 4500 : undefined

Error: undefined is not greater than 4500 : undefined

    at Error (<anonymous>)

    at doassert (src/mongo/shell/assert.js:11:14)

    at Function.assert.gt (src/mongo/shell/assert.js:295:5)

    at /data/mci/shell/src/jstests/core/explain2.js:24:8

2015-04-24T09:42:37.311-0400 E QUERY    Error: undefined is not greater than 4500 : undefined

    at Error (<anonymous>)

    at doassert (src/mongo/shell/assert.js:11:14)

    at Function.assert.gt (src/mongo/shell/assert.js:295:5)

    at /data/mci/shell/src/jstests/core/explain2.js:24:8 at src/mongo/shell/assert.js:13

failed to load: /data/mci/shell/src/jstests/core/explain2.js